Precipitation Using Organic Substances

By addition of up to 80% (v/v) of organic solvents such as methanol, ethanol, or acetone to protein solutions, a formation of precipitates occurs. Varying the solvent concentration allows a fractionation as described for salting out. Because organic solvents tend to denature proteins at temperatures above 10 °C, precipitation and all further steps have to be performed at 0 °C or below. Some buffer salts may precipitate also at elevated concentrations of organic solvents therefore, the ionic strength of buffers should not be above 0.2. 

After collection of the precipitate by centrifugation, the pellet is washed and resuspended in a suitable buffer of appropriate volume. Insoluble, denatured material is removed by centrifugation or filtration. 

Complete precipitation by PEG needs 30 min to 15 h at 0 ° C. The precipitate is collected by centrifugation or filtration on glass fiber filters (e.g., Whatman GF/A). Dissolve the precipitate in ddHaO and dialyze against an appropriate buffer. An increase in volume occurs during dialysis. 

---

The use of supercritical fluid carbon dioxide to extract low levels of organic substances from water was investigated for 23 different compounds. In general, compounds that were volatile and/or not highly soluble in water were readily extracted under the conditions used. Compounds of higher water solubility did not show evidence of extraction. In addition, those materials that tended to precipitate or form more soluble species under acidic conditions were not extracted. 

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

Ruthenium tetroxide is permanent when kept in sealed tubes in the dry state and protected from light. Exposed to light it assumes a brown colour, but the brown product is soluble in alkali, yielding a ruthenate. Presumably the coloration is due to partial reduction. Owing to its ready reduction by organic substances whereby a black precipitate of finely divided ruthenium is obtained, potassium per-ruthenate has been found useful for histological microscopy. ... 

Perchloric or sulphuric acid is usually used in the distillation. HCIO4 is more suitable since it neither complexes nor precipitates metal ions. However, perchloric acid must not be used in the presence of organic substances for fear of explosion. When HF is distilled from sulphuric acid medium, minute amounts of H2SO4, carried over to the distillate, can interfere in the determination of fluoride by complexing Zr or Th if these are used as reagents. The sample volume can be reduced by evaporation from a platinum crucible, after the sample has been made slightly alkaline. 

Various approaches have been developed based on the feedback mode. Besides driving irreversible electrochemical surface processes such as the dissolution of metals and semiconductors, the SECM has been used for driving acid-base reactions, generating radicals that deactivate biological and organic substances, and shifting precipitation reactions that affected the dissolution of salts. 

In tile test bromide is precipitated as silver bromide, and the precipitate s reaction toward nitric acid and ammonia is used to rule out other silver precipitates. Since a very high number of elements and also organic substances give a precipitate with silver nitrate, the description of selectivity below is not meant to be conclusive. It mainly deals with the other halogens and a few other inorganic cations. Precipitation with silver nitrate is also used for test (a) of 3.16. Chloride and test (a) of 3.19. Iodide, so the difference between the tests of these is dealt with in more detail. 

Action of Salts, Antiseptics, Alkaloids and of Other Organic Substances.—In the action of inorganic salts on peptonization, different factors must be considered. Certain salts precipitate albumin in great quantities but even before the quantity used is sufficient to produce predpitation, these already exert an effect on the albuminoid by changing its sensitiveness towards the proteolytic enzyme. The influence of the salts on the digestion is exerted in this case by an indirect method. It is not that the pepsin has been retarded, but rather that the protein substance has become less susceptible. Thus it is that in the presence of certain quantities of NaCl, pepsin with difficulty digests albumin, but a solution of pepsin with addition of sea salt appears very active after dialysis. 

A second liquid is sometimes added to a solution to reduce the solubility of the solute, cause its precipitation/crystallization and maximize the yield of product. It is necessary, of course, for the two liquids (the original solvent and the added precipitant) to be completely miscible with one another in all proportions. The process is commonly encountered, for instance, in the crystallization of organic substances from water-miscible organic solvents by the controlled addition of water. The term watering-out is often used in this connection. This approach is also used to reduce the solubility of an inorganic salt in aqueous solution by the addition of a water-miscible organic solvent in salting-out precipitation processes (section 7.2.5). 

---